In 30 seconds
- Yes, you can take it fasted, including if you train first thing without breakfast.
- In a study of 8 men, with insulin at fasting levels muscle carnitine did not change; with insulin raised, it rose from 22.0 to 24.6 mmol/kg.
- The trials that loaded the muscle gave it with 80 g of carbohydrate twice a day: 21% more muscle carnitine in 24 weeks.
- On rest days, or if your stomach notices it, take it with a carbohydrate meal instead: that is what the Australian Institute of Sport advises.
Yes, you can take L-carnitine on an empty stomach, including if you head out first thing without breakfast. But the studies suggest your muscles take it up better with a carbohydrate meal, so on rest days you are better off taking it with food.
Here is why, what the studies found and how to fit it around your day.
Fasted, before training: yes
It is one of the most common uses.
As a stimulant-free fat burner, it contains no caffeine or synephrine. You take it first thing, on an empty stomach, without adding stimulants before breakfast. If you also have a coffee before heading out, we explain how to combine them in L-carnitine and coffee.
And does training fasted help you lose more fat? We go through all the studies in fasted cardio.
Why the studies give it with carbohydrate
According to physiology studies, L-carnitine is what carries long-chain fatty acids into the mitochondria, where they are used for energy 1. You will find the diagram in what L-carnitine is good for.
The hard part is getting it to the muscle. Only 14-18% of the L-carnitine in a supplement is absorbed, compared with 63-75% of the L-carnitine in food 1. And once it is in the blood, your muscles do not take it up equally at all times.
The key is insulin, the hormone that rises when you eat carbohydrate. In one study, 8 healthy men received intravenous L-carnitine for 5 hours, on two occasions 2:
- With insulin at fasting levels, muscle carnitine did not change: from 22.4 to 22.7 mmol/kg.
- With insulin raised, it rose from 22.0 to 24.6 mmol/kg, an increase of about 12%.
| Study | How it was taken | Result |
|---|---|---|
| Stephens et al., 2005: 8 healthy men | Intravenous L-carnitine for 5 hours, with insulin at fasting levels | Muscle carnitine did not change: from 22.4 to 22.7 mmol/kg |
| The same study | Intravenous L-carnitine for 5 hours, with insulin raised | It rose from 22.0 to 24.6 mmol/kg |
| Wall et al., 2011: 14 men, 24 weeks | 1.36 g of L-carnitine with 80 g of carbohydrate, twice a day | 21% more carnitine in the muscle |
| Stephens et al., 2013: 12 men, 12 weeks | The same regimen, compared with the same carbohydrate without L-carnitine | Carbohydrate alone added 1.8 kg of fat; with L-carnitine, there was no change |
Sources: Stephens et al., 2005; Wall et al., 2011; Stephens et al., 2013. Muscle carnitine in mmol per kilo of dry muscle.
What carbohydrate and consistency achieved
The longest trial lasted 24 weeks, with 14 active men. All of them took 80 g of carbohydrate twice a day: some on its own and others with 2 g of L-carnitine tartrate, which provides 1.36 g of L-carnitine. In the L-carnitine group, muscle carnitine rose by 21%; in the carbohydrate-only group, it did not change 3. It is an effect that takes months: we look at it week by week in how long L-carnitine takes to work.
And it showed in training 3:
- During light exercise, at 50% of their maximum capacity, they used 55% less muscle glycogen than the carbohydrate-only group.
- In a 30-minute all-out test, their performance was 11% better than at the start.
- Body weight: the carbohydrate-only group gained 2.4 kg; the L-carnitine group stayed the same.
A second trial repeated the regimen for 12 weeks with 12 men. With L-carnitine, muscle carnitine rose by 20% and energy expenditure by 6%. With the same carbohydrate but no L-carnitine, they gained 1.9 kg of body weight and 1.8 kg of fat; with L-carnitine, neither changed 4.
That is why the Australian Institute of Sport recommends taking each dose with a carbohydrate-containing meal, to help the muscle take it up 5.
With carbohydrate and consistency
What was measured when taken with carbohydrate
When to take it, depending on your day
The pre-training serving stays where it is, whether you have eaten or not. The rest of the time, make use of your meals.
| Your day | When | Why |
|---|---|---|
| You train first thing, without breakfast | 20 ml, 30 minutes before training | It is what the label says, and it contains no caffeine or synephrine |
| You train after eating | 20 ml, 30 minutes before training | You arrive with a meal behind you and higher insulin |
| Rest day | 20 ml with a carbohydrate meal | This is how it was taken in the studies that loaded the muscle |
| Your stomach notices it when fasted | With a meal, including before training | Taking it with food helps avoid digestive discomfort |
Our own summary, based on Wall et al., 2011, and the Australian Institute of Sport.
The studies used a lot of carbohydrate: 80 g per dose 3 5. In everyday life, the practical approach is to take it with one of your main meals on the days you do not train.
Their regimens added up to about 2.7 g of L-carnitine a day, in two doses 3. If you are unsure about the exact timing, we cover it in L-carnitine: before or after training?.
If it does not agree with you on an empty stomach
At around 3 g a day, nausea, vomiting, abdominal cramps, diarrhoea and a fishy body odour can appear 1. The Australian Institute of Sport notes that L-carnitine can cause mild digestive discomfort and that, as with other supplements that do, it is best taken with food 5.
If your stomach notices it when you take it fasted, move it to after a meal, even if that is before training. And do not double the dose on the label. You will find the other precautions in L-carnitine side effects.
Does it break a fast?
It depends on why you are fasting:
- If your fast is strict, for medical or religious reasons, stick to water and keep L-carnitine for your mealtimes.
- If you are simply training before breakfast, it fits: that is exactly when the label says to take it, 30 minutes beforehand.
What the studies say about weight
L-carnitine is one of the stimulant-free fat burners with figures against placebo. In a meta-analysis of 9 trials with 911 adults, those who took it lost 1.33 kg more weight than the control group and lowered their body mass index by a further 0.47 points 6.
It builds on the foundations: a moderate deficit, enough protein and strength training. Work out your calories with the calorie deficit calculator and compare it with the rest in fat burners: which ones work, according to the studies.
If you are pregnant, take medication or have kidney or thyroid disease, check with your doctor first.
Frequently asked questions
Can you take L-carnitine on an empty stomach?
Yes. According to the studies, muscle takes it up better with carbohydrate, so on rest days it is worth taking it with a meal.
Is it better to take L-carnitine fasted or with food?
For your muscles, with food. In one study, with insulin at fasting levels muscle carnitine did not change, and with insulin raised it went up. The trials that increased it gave it with 80 g of carbohydrate twice a day.
Can I take L-carnitine before fasted cardio?
Yes, it is one of the most common uses: 20 ml 30 minutes before walking, running or cycling.
Can L-carnitine upset your stomach when taken fasted?
For some people, yes. At around 3 g a day, nausea, abdominal cramps and diarrhoea have been described, and the Australian Institute of Sport advises taking it with food if this happens to you.
Does L-carnitine break a fast?
It depends on your fast. If it is strict, for medical or religious reasons, stick to water and keep L-carnitine for your mealtimes. If you are simply training before breakfast, it fits: that is exactly when the label says to take it.
Sources
- Office of Dietary Supplements, National Institutes of Health. Carnitine: fact sheet for health professionals. View publication
- Stephens FB, Constantin-Teodosiu D, Simpson EJ, Laithwaite D, Greenhaff PL. Insulin stimulates L-carnitine accumulation in human skeletal muscle. J Physiol. 2005;565P:C41. View publication
- Wall BT, Stephens FB, Constantin-Teodosiu D, Marimuthu K, Macdonald IA, Greenhaff PL. Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. J Physiol. 2011;589(4):963-973. View publication
- Stephens FB, Wall BT, Marimuthu K, Shannon CE, Constantin-Teodosiu D, Macdonald IA, Greenhaff PL. Skeletal muscle carnitine loading increases energy expenditure, modulates fuel metabolism gene networks and prevents body fat accumulation in humans. J Physiol. 2013;591(18):4655-4666. View publication
- Australian Institute of Sport. AIS Sports Supplement Framework: carnitine. Updated March 2021. View publication
- Pooyandjoo M, Nouhi M, Shab-Bidar S, Djafarian K, Olyaeemanesh A. The effect of (L-)carnitine on weight loss in adults: a systematic review and meta-analysis of randomized controlled trials. Obes Rev. 2016;17(10):970-976. View publication
Information based on published studies. It does not replace the advice of a doctor or registered dietitian. The results describe what was measured in each study, not the properties of any specific product. A varied and balanced diet and a healthy lifestyle are important.
